How to pass a constexpr array into a function

Question

I need to perform a check on a constexpr array, but can't figure out how to pass the array into the check function.

#include <cstdlib>

constexpr int is[2] = {23, 42};

void inline check(const int (&elems)[2])
{
    static_assert(elems[0] == 23, "Does not work");
}


void bar()
{
    static_assert (is[0] == 23, "Works");
    check(is);
}

Is there a way to pass the array into the check function without loosing the constexpr property?

You can't even do it like this with a regular int. consteval might be another solution in a year or so. — nwp
– nwp, Commented May 7, 2019 at 9:37

Marek R · Accepted Answer · 2019-05-07 10:23:38Z

3

static_assert inside check depends on function parameter. It doesn't meter that you have pass constexpr argument to that function. Note that function are usually used multiple times. So in one case static_assert could be failing in other could passing. Static assert doesn't check from where function containing it was called. It must be verifiable during compilation without inspecting what is below.

Probably you need something like this:

constexpr int is[2] = {23, 42};

template<typename T>
constexpr bool firstElementIs23(const T& v)
{
    return v[0] == 23;
}

void bar()
{
    static_assert (firstElementIs23(is), "Works");
}

Live sample

answered May 7, 2019 at 10:23

Marek R

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1 Comment

Lightness Races in Orbit Over a year ago

Introducing T is a distraction but your answer is correct good job

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